regmap: add helper for per-port regfield initialization

Similar to the standalone regfields, add an initializer for the users
who need to set .id_size and .id_offset in order to use the
regmap_fields_update_bits_base API.
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20200527234113.2491988-2-olteanv@gmail.comSigned-off-by: Mark Brown <broonie@kernel.org>

include/linux/regmap.h

@@ -1134,6 +1134,14 @@ struct reg_field {
 				.msb = _msb,	\
 				}
 
+#define REG_FIELD_ID(_reg, _lsb, _msb, _size, _offset) {	\
+				.reg = _reg,			\
+				.lsb = _lsb,			\
+				.msb = _msb,			\
+				.id_size = _size,		\
+				.id_offset = _offset,		\
+				}
+
 struct regmap_field *regmap_field_alloc(struct regmap *regmap,
 		struct reg_field reg_field);
 void regmap_field_free(struct regmap_field *field);